1. Field of the Invention
This invention relates generally to rotary gas suction pumps, particularly to axial flow molecular vacuum pumps intended for generating high vacuum, and more specifically concerns a turbomolecular vacuum pump.
2. Description of the Related Art
Modern advances in science and technology necessitate high demand for a range of types and sizes of molecular vacuum pumps of various suction performance, viz., rapidity and gas compression ratio, determining the size of major working parts of such pumps.
Depending on the pumping characteristics, there are known molecular vacuum pumps having only a molecular pumping stage, and turbomolecular or combination vacuum pumps having an additional turbomolecular gas pumping stage comprising an assembly of rotor and stator arranged axially with the rotor and stator of the molecular gas pumping stage at the gas suction side. Secured alternately at the rotor and stator of the turbomolecular gas pumping stage are bladed wheels and bladed disks having blades thereof positioned at an angle to each other. The blades of the bladed wheels are inclined to planes perpendicular to the axis of rotation of the rotor, whereas the flow sections of interblade passages reduce from the bladed wheel at the gas suction side to the bladed wheel at the gas pressure side. These turbomolecular vacuum pumps are characterized by sufficiently high rapidity, although they are overcomplicated structurally.
The operating principle of a turbomolecular vacuum pump resides in that molecules of gas collide with blades of the rotating bladed wheel and receive an impulse which adds a tangential velocity component in the direction of rotation of the bladed wheel to the inherent thermal velocity of the molecules. Multiple collisions of the gas molecules with the rotor blades turn random motion of the molecules into ordered motion in a direction from the gas suction side toward the gas pressure side resulting in evacuation of the gas molecules.
In the flow of molecules the average free travel of a gas molecule is greater than the distance between the adjacent blades, and therefore the molecules tend to collide with the rotor blades more often than with one another.
Efficiency in operation and rapidity of the turbomolecular vacuum pump depends on what part of gas molecules is conveyed through the bladed wheels and bladed disks from the gas suction side to the gas pressure side.
There is known a turbomolecular vacuum pump comprising a hollow stator the axial interior hole of which accommodates a rotor with at least two bladed wheels having bladed disks interposed therebetween and secured on the stator, the flat blades of these bladed disks being arranged at an angle to the flat blades of the bladed wheels of the rotor spaced equidistantly about the circle of a hub of the corresponding bladed wheel so that the flow sections of passages between the planes of the adjacent blades facing each other reduce from the bladed wheel at the gas suction side to the bladed wheel at the gas pressure side, the planes thereof being inclined to planes perpendicular to the axis of rotation of the rotor in the direction of its rotation.
The blades of the bladed wheels are disposed at the hubs so that lines of intersection of the planes of each of the blades with planes perpendicular to the axis of rotation of the rotor extend radially about the circumference of the hub.
Of decisive importance for the suction characteristics of the turbomolecular vacuum pump is the dependence between the suction rapidity and gas compression ratio. This dependence is determined by the geometry of the bladed wheels and bladed disks, as well as the size of the major structural elements of the pump.
In the known turbomolecular vacuum pump molecules following the mirror reflection law and moving in planes tangent to circles coaxial with the circle of the hub and intersecting the planes of the blades are caused to obtain during collision therewith a tangential velocity component, whereby the molecules of gas partially return to the enclosure being evacuated and partially collide again in the same planes obtaining the same impulse. This, however, affects the rapidity of the turbomolecular pump. In addition, a backflow of dispersed molecules of gas in the clearance between the rotor and stator takes place.